feat: theme family system with 12 color palettes + NavRail redesign
Theme system:
- Two-layer architecture: mode (light/dark/system via next-themes) + theme family (color palette via CSS variables)
- 12 theme families: Default, GitHub, Rosé Pine, Tokyo Night, Everforest, Nord, Kanagawa, Night Owl, Poimandres, Vesper, Horizon, SynthWave '84
- Per-family code block highlighting for Prism, HLJS, and Shiki (no more generic oneLight/oneDark fallback)
- Anti-FOUC: synchronous script in <head> reads localStorage and sets data-theme-family before paint
- Theme JSON files in themes/ directory, auto-discovered by server-side loader
- OKLCH color space for all values; CSS specificity html[data-theme-family] beats :root
UI changes:
- NavRail: moved Settings to bottom (replacing theme toggle), removed light/dark toggle button
- Settings: merged General controls into one card with dividers; Appearance section with live Shiki code preview
- Replaced hardcoded blue-500/blue-600 with primary/primary-foreground across 19 components
- Appearance settings: pill mode selector (light/dark/system), theme family dropdown, live code preview
New files:
- src/lib/theme/{types,loader,render-css,context,code-themes}.ts — core theme infrastructure
- src/components/layout/ThemeFamilyProvider.tsx — client context provider
- src/components/settings/AppearanceSection.tsx — appearance settings UI
- src/hooks/useAppTheme.ts — combined theme hook
- themes/*.json — 12 theme family definitions
- docs/handover/theme-system.md — handover documentation
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
